* Refcount leak. It was just a reference to Py_None, but still.

* Refcount leak.  It was just a reference to Py_None, but still.

* Allow the 3rd argument to generator.throw() to be None.
  The 'raise' statement does the same, and anyway it follows the
  general policy that optional arguments of built-ins should, when
  reasonable, have a default value specifiable from Python.